The Romans considered Opal
to be a symbol of hope and purity. To the early Greeks
Opal embodied the powers of foresight and prophecy.
Arabs believed opal achieved their unique color play or opalescence by falling from heaven in
flashes of lightning. Opal gets its name from Opalus,
the ancient Latin name which was probably derived from the
Sanskrit word upala, meaning precious stone.
The Greek word opallios literally means "to
see a change of color."

Opal
comes in a wide variety of types and colors from many
locations world
wide.
Opal has almost as many names as
locations. Some Opal names are in reference to their
locality such as Lightning Ridge, Coober Pedy and Virgin
Valley. Other names are due to their color or type such
as Cherry, Common, Girasol, Hyalite, Water, Fire, Jelly,
Milk, Black, Crystal, Hydrophane, Moss and Harlequin.
Many Opals are transparent but others are only known
to be opaque and are made into beautiful cabochons
with amazing color play like Australian Opals. Australian Opal
cabochons can have translucent to opaque white, gray
or black body colors with beautiful color play on their
surface.

Transparent
opals may have solid body colors with no color play or
they may be lightly colored or colorless but show beautiful
internal color play with all the colors of
a rainbow. These may be called Contra Luz or Precious
Opal because of their amazing internal
color play when lighted from behind. Some Opals from
Ethiopia have both strong body colors and beautiful
internal color play. Faceted Brazilian and Mexican Opals are
usually found in vivid colors of yellow, orange and
red. They are often called Fire Opals for their
body colors resembling the colors of fire, not due to internal
fire or color play. Colorless Opals are often called
Jelly, Water, Crystal, Girasol or Hyalite Opal.
Hyalite Opal from Madagascar is
colorless with a soft, beautiful glow. There are
also opaque Opals with no color play just beautiful
pastel colors like pink Opal from Peru; blue Opal from
Owyhee, Oregon;
and
pale green Opal from Siberia and Africa. The USA has several locations known for beautiful
Opals including Idaho, Oregon and Nevada.

Although
Opal is amorphous (no crystal structure) it does have
a structure of sorts. Opal is composed of tiny spherical
particles as a solidified gel also containing 5-10%
water. Random chains of silicon dioxide and water are
arranged into tiny spheres. In most Opals these spheres
are irregular in size and concentration, kind of jumbled.
But in Opals with color play, such as Contra-luz or
Precious Opal, there are many pockets of spheres that
are of approximately equal size and have a regular concentration,
or structure. The water contained in these pockets of
somewhat organized structure has the effect of diffracting
light at various wavelengths creating a display of different
colors. This effect is similar to when a rainbow is
created from the sun shining through water particles
in the air. Each pocket may produce a different color
or different intensity of color depending on their angle
in the composition and the angle at which the gem is
viewed. This effect creates the beautiful color play
or opalescence that makes some Opals much more valuable
than others.

Since
Opal contains water, it may also dehydrate causing
it to craze, crack or become very cloudy or even opaque.
This may happen as soon as it is removed from the ground
or years later. When this happens, a beautiful Opal
may become ugly and worthless.

Of
all the Opal taken out of the ground, 95% is valueless
"potch" and 95% of the remainder is low quality.
Only a mere 0.25% ever makes it to market as a gem.